Utility of portable monitoring in the diagnosis of obstructive sleep apneaU Krishnaswamy1, A Aneja1, R Mohan Kumar2, T Prasanna Kumar1
1 Department of Respiratory Medicine, MS Ramaiah Medical College, Bangalore, Karnataka, India
2 Megachips India, Bangalore, Karnataka, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.166509
Source of Support: None, Conflict of Interest: None
Obstructive sleep apnea (OSA) is a common but underdiagnosed sleep disorder, which is associated with systemic consequences such as hypertension, stroke, metabolic syndrome, and ischemic heart disease. Nocturnal laboratory-based polysomnography (PSG) is the gold standard test for diagnosis of OSA. PSG consists of a simultaneous recording of multiple physiologic parameters related to sleep and wakefulness including electroencephalography (EEG), electrooculography (EOG), surface electromyography (EMG), airflow measurement using thermistor and nasal pressure transducer, pulse oximetry and respiratory effort (thoracic and abdominal). Multiple alternative and simpler methods that record respiratory parameters alone for diagnosing OSA have been developed in the past two decades. These devices are called portable monitors (PMs) and enable performing sleep studies at a lower cost with shorter waiting times. It has been observed and reported that comprehensive sleep evaluation coupled with the use of PMs can fulfill the unmet need for diagnostic testing in various out-of-hospital settings in patients with suspected OSA. This article reviews the available medical literature on PMs in order to justify the utility of PMs in the diagnosis of OSA, especially in resource-poor, high-disease burden settings. The published practice parameters for the use of these devices have also been reviewed with respect to their relevance in the Indian setting.
Keywords: Obstructive sleep apnea (OSA), polysomnography (PSG), portable monitors (PMs), sleep
Obstructive sleep apnea (OSA) is an important, yet inadequately diagnosed public health problem that is characterized by repetitive collapse of the upper airway during sleep.  It has been reported that OSA affects up to 5% of the world's population  with a prevalence of prevalence of 4% in males and 2% in females  aged between 30 years and 60 years of age. The available literature on the prevalence of this disorder in the Indian population is limited. A study performed by Sharma et al. in an urban setting estimated the prevalence of OSA to be 13.7%.
OSA is a predisposing factor for multiple comorbidities such as hypertension, cardiovascular consequences (including sudden death), , ischemic heart disease, stroke ,,,,, , and uncontrolled diabetes. An increased risk of road traffic accidents has also been reported in patients suffering from OSA.  The awareness about this disorder and its associated outcomes is on the rise among patients and clinicians. This has resulted in ever-increasing referrals to sleep clinics for diagnostic evaluation but the infrastructure to support the demand for rising sleep studies has not escalated commensurately.  The "gold standard" test for diagnosis of sleep disorders is a conventional, fully supervised, in-laboratory polysomnography (PSG).  With the limited availability of accredited sleep laboratories, certified sleep technologists, and complete polysomnography (CPSG) equipment in many countries, long waiting times and high costs of testing are inevitable. 
In view of the above mentioned reasons, several alternative diagnostic tools such as sleepiness questionnaires, clinical decision rules,  portable monitors (PMs), actigraphs, and peripheral arterial tone (PAT) monitors , have been developed for the evaluation of OSA. Of these, portable monitors have been evaluated and accepted for selected patient groups in clinical guidelines in various countries. ,
This article reviews recent guidelines and published literature on the clinical use of portable monitoring devices to diagnose OSA and attempts to define the role of portable monitors, especially in high-disease burden, resource poor settings.
A search of the available literature was done using the keywords "portable monitoring in obstructive sleep apnea" with filters over last 10 years in humans. The Medline search generated 125 results over the last 10 years. This included 2 systematic reviews and meta-analysis, 3 guideline statements, 16 review articles, 16 randomized controlled trials, and 24 validation studies. Sixty-four articles included expert opinions and cross-sectional studies. In addition, the search was supplemented with data from cross references cited in the bibliography of relevant articles.
The present review has been organized into the following sections:
Sleep testing devices were initially classified by the American Academy of Sleep Medicine into level 1 to level 4 devices based on the type of leads and setting of the device used  [Table 1]. Among these, portable monitoring devices were categorized as level 2, level 3, and level 4 sleep monitoring devices. These monitors have limited number of channels and are used for the diagnosis of certain sleep disorders such as OSA, insomnia, and restless legs syndrome. Recent technical advances have resulted in the design of a variety of compact PM devices that are battery-operated and easy to hook up (wearable). Many of these devices also have wireless monitoring capability. These devices are being proposed as feasible options for out-of-lab diagnosis of OSA. The various modes for portable sleep monitoring based on location, type of sleep study, and technician attendance have been depicted in [Table 2]. However, there are various models of such out of center (OOC) devices, incorporating different combinations of sensors for monitoring. Hence, a blanket acceptance or rejection of such devices has been a problem faced by sleep societies around the world. Recently, a system has been introduced categorizing these devices based on measurements of sleep, cardiovascular, oximetry, position, effort, and respiratory (SCOPER) parameters.  Using the SCOPER categorization, available PM devices with appropriate literature and validation studies have been categorized into a common scheme and devices are judged on whether they can produce a likelihood ratio (LR+) of at least 5 and a sensitivity of at least 0.825 at an in-laboratory apnea-hypopnea index (AHI) of 5. 
PMs have been used as diagnostic tools for OSA for over two decades.  Several of these devices have been compared with the gold standard PSG in patients with suspected OSA. However, there is a paucity of data comparing the available PM devices with one other. Some of the PMs that have been validated against PSG include ApneaLink,  Edentec, ,, PolyG,  AutoSet, ,,,, Embletta,  Sibel home,  Bedbugg,  NovaSom,  WatchPAT, ,,, SNAP,  SOMNOcheck,  Stardust II,  Apnomonitor,  and Apnea Risk Evaluation System (ARES)  [Table 3]. The results of the studies conducted with some of these devices are discussed in detail below.
Studies conducted to validate PM
Level 4 devices are the simplest forms of PMs and consist of a nasal cannula to monitor airflow and a finger probe to record oxygen saturation. Milton et al. evaluated the sensitivity and specificity of a Level 4 device, the ApneaLink™  (ResMed Corporation) as a screening tool for sleep apnea. The AHI obtained using this device in the laboratory and at home was compared to attended sleep-laboratory PSG in fifty-nine patients with type 2 diabetes mellitus. The study demonstrated a high sensitivity and specificity (91% and 95%, respectively) of the at-home ApneaLink AHI with the simultaneous PSG at all AHI levels, the best results being at an AHI of ≥15 events/ h. The AHI comparison from the home and laboratory PM studies also demonstrated good sensitivity and specificity at AHI levels of ≥15 and ≥20 events/ h (sensitivity 76% and specificity 94% for both).
The same device was also compared with a level 1 device (Alice 4) for diagnostic accuracy by Ng et al. in 50 patients with suspected OSA. They observed a close correlation (r = 0.978) between AHI obtained by the two devices at AHI >10/h as well as >20/h. Further, the correlation between PSG AHI and oxygen desaturation index by ApneaLink was strong (r = 0.895) thereby drawing the conclusion that the ApneaLink device was highly sensitive and specific in quantifying AHI for screening and diagnostic purposes when access to PSG was limited.
Another level 3 sleep monitor, Apnoeascreen II (hooked up by a technician) was compared with in-laboratory PSG by Quintana-Gallego et al. in 75 patients with stable heart failure. Using AHI cutoff points of >5, >10, and >15, the authors generated diagnostic accuracies of 78.6%, 84%, and 80%, respectively. However, they reported an overall diagnostic failure rate of 8% in home studies.
While validation studies of several such monitors have been conducted, most studies comprised a small number of subjects. The details of studies performed with such devices are summarized in [Table 3].
Home sleep studies have been evaluated with simultaneous in-laboratory PSG by many researchers. Iber et al. compared in-laboratory and home monitoring in 76 participants using a level 2 device (Compumedics, Australia) using the Sleep Heart Health Study (SHHS) methodology. They reported that the median sleep time and sleep efficiency were better in home sleep studies. Further, median RDI values were comparable between at-home and in-laboratory measurements. They suggested that patients with an RDI <20 were more likely to be diagnosed in the laboratory when compared to those with RDI >20. The misclassification rate in home studies was 22% and was hypothesized as being due to night-to-night variability of OSA. Other studies ,,,, have also highlighted the night-to-night variability ranging from 10% to 20% in attended as well as unattended studies.
Dingli et al. evaluated synchronous recording with PM and traditional in-laboratory PSG followed by a home sleep study with the Embletta device (Flaga, Iceland), a level 3 monitor. The home-based studies showed an overall good agreement with laboratory-based PSG outcomes (kappa coefficient: 0.54; P - 0.001). Apart from this, home screening resulted in 42% saving in diagnostic costs. Thus, home monitoring was suggested as a reliable and cost-effective method for screening in patients with suspected OSA.
The AASM guideline published in 2007 defines indications for unattended portable monitoring in specific patient populations with high suspicion of OSA.  According to the Indian Initiative on Obstructive Sleep Apnea (INOSA) guidelines, a laboratory-attended PSG is not necessary in all patients suspected to have OSA. Portable monitoring or out of center testing (OCST) with type 3 or type 4 devices (that should at least include airflow, oxygen saturation, and respiratory effort) is adequate for diagnosis. This is acceptable in conjunction with comprehensive sleep evaluation and in patients with high pretest probability of moderate to severe OSA without comorbid sleep disorders or medical disorders such as pulmonary disease, neuromuscular disease, or congestive heart failure. 
According to INOSA guidelines, comprehensive evaluation and assessment of pretest probability can be performed using questionnaires such as Epworth Sleepiness Scale (ESS), Berlin questionnaire, modified Berlin questionnaire and STOP-BANG questionnaire. Out of these, STOP-BANG is the most recommended screening tool before portable monitoring due to the ease of administration and high sensitivity.
Although several comparative studies between PM and level 1 sleep studies have demonstrated a high sensitivity for PM, it is noteworthy that these studies predominantly concentrate on diagnostic accuracy and reliability. Translation of results derived from PM into continuous positive airway pressure (CPAP) prescription is an important consideration. This is especially important in settings where low cost home testing is a preferable alternative to in-laboratory PSG. Some prospective studies pertaining to this aspect of portable monitoring have been discussed below.
Rafael and coworkers  studied 55 patients wherein a therapeutic decision (use of CPAP) made with unattended PM device was compared with that made after a subsequent PSG. Patients were equally subdivided into the technician hookup (group 1) and self-hookup groups (group 2) at the time of initial PM monitoring. The authors reported that 16% of the recordings among the home study group were inconclusive. In 75% of the interpretable home studies, the diagnosis was seen to correlate with in laboratory PSG; this figure rose to 89% on exclusion of inconclusive home studies. Among the two subsets of home study, data were not interpretable due to artifacts in 7% of the patients in group 1 and 33% in group 2 (P < 0.05). CPAP decision disagreement between home and lab studies occurred in about one third of patients. Cost effectiveness analysis showed reduction in study costs at home compared to in lab study. Thus, this study projected home sleep studies with technician hook-up as a viable diagnostic modality to decide about CPAP prescription.
In a similar study, Parra et al. tested 89 patients with suspected OSA in two settings, in the sleep laboratory using level 1 sleep study and at the patient's home using a portable monitor. In the latter setting, 50 patients were assisted by a technician and 39 patients set up the equipment themselves. An acceptable agreement was obtained between the AHI measured by full-PSG and PM. The clinical therapeutic decision taken after PM agreed with that taken with full-PSG in 89% of patients. Apart from this, the domiciliary studies were about three times less costly than level 1 PSG. They concluded that patients with suspected OSA should initially be studied in a home setting with a PM. Parra et al. also noted that patients who were located far away from the hospital frequently preferred to be studied in the laboratory than at home. These findings were further validated by other studies.  However, in contrast there are two studies with a similar design that have found a higher frequency of unacceptable studies with PMs. , The discrepancy with respect to error rates and acceptability of PMs among the various studies cited above can be explained in relation to the differences in technical specifications, data analyzed, sensors used in various PM devices, diverse patient groups and preferences, and study setting and availability of technicians for hookup at home.
Since continuous positive airway pressure can be titrated most effectively with monitoring in a sleep laboratory,  an attended in-laboratory CPAP titration study/split night study was also necessary conventionally after a diagnosis of OSA was made. However, nowadays the validation and clinical use of the auto-titrating PAP device minimizes the need for an in-laboratory titration study for patients with moderate to severe OSA. This is feasible since these devices can be used in an unattended way to determine fixed CPAP treatment pressure for such patients who do not have significant co morbidities such as congestive cardiac failure, chronic obstructive pulmonary disease, central sleep apnea syndrome, and hypoventilation syndrome. 
Further, in patients with more than one sleep disorder such as OSA and insomnia, in patients with problems such as overlap syndrome (where suboptimal levels of CPAP can actually worsen hypoxia) as well as patients underdiagnosed by PM, home studies may be followed by repeat type I test. Apart from this, knowledge of autotitrating CPAP devices may not be uniform, especially with respect to known contraindications in certain special populations (heart failure), limitations (nasal obstruction), and variability in device performance, and air leak between manufacturers. 
As opposed to the global data described above, awareness about sleep disorders and scientific research in sleep medicine in the Indian subcontinent has demonstrated a gradual upward trend only since the past two decades. The paucity of trained sleep technologists and physicians, financial considerations, and an unknown disease burden has led to a widespread use of PMs in both clinical and research settings. In a study by Udwadia et al.,  the prevalence of OSA was estimated using technician hookup-based PM (Compumedics P series). OSA prevalence in middle-aged urban Indian men was estimated to be 19.5%.
It is evident from the literature reviewed in the preceding paragraphs that there is considerable heterogeneity in the applicability and modes of portable monitoring for the diagnosis of OSA.
However, there is compelling evidence to suggest that home sleep studies have evolved as a viable and more economical mode of diagnosing OSA worldwide. When equipment with multiple channels are used, the involvement of a sleep technician for hookup in the patient's home has been shown to be a good intervention to improve the accuracy and reliability of testing. The inferred merits and demerits of PM are summarized in [Table 4].
Practice parameters for PSG were first published by the American Sleep Disorders Association in 1994.  On the basis of the then available literature on PM, these guidelines considered that PM had inadequate sensitivity for routine diagnostic testing. Subsequent to these guidelines, several systematic reviews and meta-analyses regarding the use of portable devices were published.
In 2007, the American Academy of Sleep Medicine (AASM) updated practice parameters for the use of unattended PMs in the diagnosis of OSA in adult patients and since its publication, further studies have already been conducted adding to the pool of evidence. The major recommendations in this statement are hereby reviewed in the context of their adaptability in the Indian setting [Table 5]. Recently published Indian guidelines (INOSA) accept PM (type 3 and 4) as a useful, cost-effective, convenient, and speedy method of diagnosis after careful patient selection and by a trained sleep physician.
A flowchart to guide selection of patients for portable sleep studies is depicted in [Figure 1].
A detailed evaluation for pretest probability (risk stratification, screening with sleepiness scales, and clinical exam, especially upper airway evaluation) in conjunction with portable monitoring has been recommended to obviate the need for level 1 and level 2 sleep studies in patients with OSA. This holds special importance, especially in resource poor settings. The published practice parameters for PM are relevant and applicable in the Indian setting. In conclusion, a greater adoption of PMs by trained sleep physicians would enable the timely diagnosis of OSA with reduced cost and waiting time.
Financial support and sponsorship
This is to state that none of the authors have received any form of sponsorship, support, or funding in the compilation and drafting of this review article.
Conflicts of interest
There are no conflicts of interest.
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]